1. Technical Field
Exemplary aspects of the present disclosure generally relate to a transfer device and an image forming apparatus including the transfer device, and more particularly to an image forming apparatus such as a copier, a facsimile machine, and a printer.
2. Description of the Related Art
There is known an image forming apparatus equipped with a transfer device that transfers a toner image from an image bearer onto a recording medium interposed in a so-called transfer nip at which the image bearer and a nip forming device contact. A transfer bias is applied to the transfer nip by a transfer bias power source to transfer the toner image from the image bearer onto the recording medium.
As an example of such a transfer device, a secondary transfer device is known to transfer secondarily a composite toner image formed on an intermediate transfer belt onto the recording medium. Initially, toner images formed on a plurality of photoconductors are transferred onto the intermediate transfer belt in primary transfer nips at which the photoconductors and the intermediate transfer belt contact such that the toner images are superimposed one atop the other, thereby forming the composite toner image. The composite toner image is then transferred secondarily from the intermediate transfer belt to the recording medium by the secondary transfer device.
The secondary transfer device includes a secondary transfer roller as a nip forming device and a secondary-transfer opposed roller. The secondary transfer roller contacts the intermediate transfer belt serving as the image bearer. The secondary-transfer opposed roller is disposed opposite the secondary transfer roller via the intermediate transfer belt and contacts the intermediate transfer belt from the back thereof. The intermediate transfer belt is interposed between the secondary transfer roller and the secondary-transfer opposed roller to form a secondary transfer nip.
When the recording medium enters and exits the secondary transfer nip, producing impact, the traveling speed of the intermediate transfer belt changes suddenly which then causes an image to be transferred onto the intermediate transfer belt from the photoconductor in the primary transfer nip to stretch or shrink undesirably. As a result, the density of toner changes at a place where the density is expected to be constant, thereby generating undesirable streaking or a so-called shock jitter.
In view of the above, a known image forming apparatus includes a contact-and-separation device that moves an intermediate transfer belt and a secondary transfer roller to contact and separate from each other. Before the recording medium enters the secondary transfer nip, an eccentric cam of the contact-and-separation device separates the secondary transfer roller from the intermediate transfer belt. Accordingly, the secondary transfer roller and the intermediate transfer belt are separated, and a certain space is formed therebetween, hence reducing shock jitter when the recording medium enters the secondary transfer nip.
Furthermore, immediately after the leading end of the recording medium enters the space, the secondary transfer roller is released by the eccentric cam and is pushed against the intermediate transfer belt by a spring. In this configuration, the secondary transfer roller contacts the intermediate transfer belt, and adequate transfer pressure is obtained at the secondary transfer nip during transfer, thereby preventing transfer failure.
After the image on the intermediate transfer belt is transferred onto the recording medium, but before the recording medium exists the secondary transfer nip, the eccentric cam of the contact-and-separation device separates the secondary transfer roller from the intermediate transfer belt by the same predetermined amount. In this configuration, the secondary transfer roller and the intermediate transfer belt are separated when the recording medium exits the secondary transfer nip, hence reducing shock jitter when the recording medium exits the secondary transfer nip.
The smaller is the transfer pressure, the smaller is the impact when the recording medium enters and exits. Therefore, if the intermediate transfer belt and the secondary transfer roller are separated completely and hence there is no transfer pressure, the recording medium can enter and exit the secondary transfer nip without producing shock jitter.
However, when the intermediate transfer belt and the secondary transfer roller come in contact again, the secondary transfer roller strikes the intermediate transfer belt due to spring force of the spring, thereby producing impact. The larger is the space between the intermediate transfer belt and the secondary transfer roller, the greater is the impact. As a result, the traveling speed of the intermediate transfer belt changes suddenly, causing shock jitter.
In this configuration, the larger is the space between the secondary transfer roller and the intermediate transfer belt, the less is the impact generated when the recording medium enters the secondary transfer nip. However, the impact is greater when the intermediate transfer belt contacts the secondary transfer roller.